Automatic frequency control (AFC) systems function to frequency stabilize a source of RF energy or to maintain a frequency relationship between sources of RF energy. Generally, a difference frequency, obtained by mixing two signals, is maintained at a constant value irrespective of the absolute stability of either signal source. Many AFC systems are used with superheterodyne type television receivers and operate by controlling the frequency of the local oscillator in the receiver. Such systems operate on the received signal and can function only after the receiver has been approximately tuned to an incoming signal by some other means. There is generally a frequency detector for producing an error voltage which is proportional to the difference between the developed IF (difference) frequency and a predetermined reference frequency. The error voltage is supplied to a control circuit to change the local oscillator frequency in the correct direction to reduce the error voltage. There must always be a small error or "offset" to provide a correction voltage, but the magnitude of error required may be decreased by increasing the amplification of the AFC loop.
Two criteria for AFC systems are pull-in range and hold-in range. Assuming the proper signal-independent setting of the local oscillator frequency, the pull-in range is the maximum frequency differential for which the system will move the local oscillator frequency to tune the receiver to the incoming frequency. The hold-in range is normally larger and is the maximum frequency differential between the oscillator and incoming signal for which the AFC system will maintain tuning, barring interruptions of the signal.
An additional criterion for AFC systems is the pull-in ratio which is the ratio of the frequency movement or displacement to the frequency differential remaining after the AFC system has acted. For example, if the received signal frequency is 500 KHz away from the oscillator frequency, an AFC system with a pull-in ratio of 10 to 1 (normal for television receivers) would move the oscillator tuning frequency to within 50 KHz of the received signal.
As mentioned, conventional AFC systems can never tune the oscillator exactly to the incoming signal and a small offset error always remains. The magnitude of the offset error can be diminished by increasing the loop gain but this may well be at a loss of stability in the tuning system. Likewise, the pull-in ratio for a conventional AFC system is limited by the gain of the AFC loop.
In an AFC system for a varactor diode tuner a separate correction voltage is added to the tuning voltage supplied by the approximate, or coarse, tuning source to achieve tuning control. There is a time delay between detection of the received signal and the application of the correction voltage. With a normal AFC system, "overshoot" can occur in which the AFC system drives the oscillator past the correct frequency. The detector then responds by reversing the voltage correction in an effort to drive the oscillator back. If the loop gain is very high an oscillatory condition can result. Therefore, achieving very high pull-in ratios by increasing AFC loop gain is normally not practical.
In conventional AFC systems the magnitude of the correction voltage is proportional to the frequency error detected, and the control voltage is added to the coarse or signal-independent tuning voltage. U.S. Pat. No. 3,697,885 to Avins describes a varactor tuning system in which a variable DC voltage provides coarse tuning. A correction DC voltage is also supplied by a frequency discriminator circuit. It should be observed that the correction voltage may be plus or minus depending upon the direction of the deviation of the IF signal and reference frequency in the discriminator. The specific AFC error signal is converted into a current which is injected into a voltage driver for equalizing the AFC loop gain over the receiver tuning range. The need for loop gain control results from the combining of separate DC voltages for tuning.
The signal-tracking means of the present invention employ a digital control concept to an AFC system. With it normal frequency detection apparatus may be used to reduce the oscillator offset error to zero. Rather than providing a separate additive correction voltage, the signal-independent tuning voltage itself is adjusted by supplying increments of charge to a tuning voltage capacitor. Thus, within the limits of device activation voltage limits, the oscillator frequency is brought into precise tuning relationship with the received signal. In this condition the signal tracking means is inoperative and uses no power, which is an added advantage.
Since the circuit embodying this invention exhibits two states, that is, it is either operating to cause tuning to within a narrow frequency difference between the IF signal frequency and reference frequency or it is inoperative, it does not exhibit a holding range in the normal sense of the term. It has rather a frequency deviation threshold beyond which it operates to bring the oscillator tuning to a point within the frequency deviation threshold region. If either the received signal or the oscillator results in a-greater-than-threshold difference between the IF and reference frequencies, the system is activated. The pull-in range for the signal tracking means of the invention may be quite similar to that for AFC systems since it uses conventional circuitry.
A distinguishing characteristic of the AFC or signal tracking system of the present invention lies in the fact that the concept of a pull-in ratio is inappropriate. For any given displacement a pull-in ratio value could be determined but this is not a parameter which characterizes the operation of the system, since, were it not for device activation voltage limits, the oscillator would be brought to its precise frequency every time. For any displacement up to the maximum pull-in range the signal tracking system of the invention will tune the oscillator to a narrow offset region defined by device deviation limits. The maximum pull-in ratio would be derived from the quotient of the pull-in frequency range divided by the narrow tuning range. This value would far exceed the stable pull-in ratio which could be achieved by increasing loop gain in a conventional AFC system.
As an example of the magnitude of the narrow offset frequency region for an embodiment of this invention, consider a frequency detector having a sensitivity of 30 volts per MHz, steering diodes having activation potentials of 0.5 volts and a pull-in range greater than 1 MHz. The corresponding offset frequency region assuming linearly in the operational region would be in the ratio of one volt (the sum of the diode activation potentials) to 30 volts which corresponds to approximately 0.33 MHz. Within the pull-in range of the system, this would mean that control can be achieved and tuning corrected to a region which is plus or minus 16.5 KHz on either side of the original undisplaced signal regardless of the magnitude of displacement.